1. Technical Field
Various embodiments of the present disclosure relate to nonvolatile memory devices and, more particularly, to nonvolatile memory devices having a wide operation range.
2. Related Art
Semiconductor memory devices are typically categorized as random access memory (RAM) devices or read only memory (ROM) devices according to data volatility thereof. The RAM devices lose their stored data when their power supplies are interrupted. In contrast, the ROM devices retain their stored data when their power supplies are interrupted. The ROM devices may also be classified as programmable ROM (PROM) devices or mask ROM devices according to data input methods, that is, data program methods. The PROM devices may be fabricated and sold out without program and may be directly programmed by customers (i.e., users) after fabrication thereof. The mask ROM devices may be programmed during fabrication thereof using implantation masks manufactured based on data requested by users. The PROM devices may include one-time PROM (OTPROM) devices, erasable PROM (EPROM) devices and electrically erasable PROM (EEPROM) devices. Once the OTPROM devices are programmed, the programmed data of the OTPROM devices cannot be changed.
N-channel transistors or P-channel transistors may be used as cell transistors of the nonvolatile memory devices, for example, the OTPROM devices. If P-channel transistors are used as the cell transistors of the nonvolatile memory devices, the P-channel cell transistors may have a turn-off state as an initial state thereof and may have a turn-on state as a programmed state thereof. A read operation of the P-channel cell transistors may be performed by sensing a voltage level of a bit line connected to a P-channel cell transistor selected from the P-channel cell transistors. In such a case, the voltage level of the bit line may be determined by a resistance ratio of a resistance of a load resistor coupled between a supply voltage line and the bit line and an equivalent resistance of the selected P-channel cell transistor. As electronic systems become scaled down, various supply voltage levels have been required to operate the nonvolatile memory devices employed in the electronic systems. In such a case, a read operation of a nonvolatile memory device employing P-channel transistors as the cell transistors may become unstable thus limiting an operation range of the nonvolatile memory device.